In recent years, with enhancement of video equipment, such as a video tape recorder or a video disk player and video software, desires for image display apparatuses capable of enjoying powerful video have increased.
Examples of video display apparatuses for spatially modulating light emitted from a light source by a transmission type or reflection type display component having a light modulating function to display video include a direct view type liquid crystal display apparatus and a projection type display apparatus. In the direct view type liquid crystal display apparatus, a liquid crystal panel is used for the display component, video displayed on the liquid crystal panel is directly viewed. On the other hand, the direct view type display apparatus, the projection type display apparatus has a primary object: to reduce the power consumption of the apparatus rather than to improve the display quality; to carry out brightness control to set the brightness of video so that the video is easily seen with respect to a screen size and under environmental illumination conditions; and to lengthen the life of the light source. And, as in the direct view type liquid crystal display apparatus, the brightness level of the light source is controlled manually by a user and, thereafter, the brightness of the light source is in a fixed state.
As described in the foregoing, in both the conventional direct view type liquid crystal display apparatus and projection type display apparatus, there have been strong demands for the improvement in the display quality over the lack of the brightness feeling in the bright scene and the floating blackness in the dark scene, as compared with the self-light emission type display apparatus, such as the CRT. However, the method of controlling the brightness of the light source which has been employed in the conventional direct view type liquid crystal display apparatus and projection type display apparatus is static fixed control and does not cope with the dynamic change in the input video signal, as described above. Therefore, the display quality in each scene of the input video signal cannot be improved.
As one method of improving the display quality, a method of dynamically changing the brightness of a light source depending on a video scene has been proposed. The method is disclosed in Japanese Patent Laid-Open Publication No. 5-127608 and Japanese Patent Laid-Open Publication No. 6-160811, for example.
Japanese Patent Laid-Open Publication No. 5-127608 has proposed a method of detecting the features of an input video signal from the maximum value and the minimum value of the input video signal, and an average value there between, to lower, when the level difference between the maximum value and the minimum value is large, contrast control to decrease signal amplification gain, while conversely raising, when the level difference between the maximum value and the minimum value is small, contrast control to increase signal amplification gain as well as to lower, when the average value between the maximum value and the minimum value is higher than a defined value previously set, the brightness of a light source to bring the brightness of a display apparatus nearer to a predetermined value.
On the other hand, Japanese Patent Laid-Open Publication No. 6-160811 has proposed a method of detecting the maximum value of an input video signal, to raise the brightness of a light source when the maximum value is high, while lowering the brightness of the light source when the maximum value is low, and to make the amplitude of a brightness signal in a case of a low maximum value smaller than the amplitude of the brightness signal in a case of a high maximum value, thereby increasing the relative contrast ratio in a case where the maximum value is high and a case where it is low.
A technique proposed in Japanese Patent Laid-Open Publication No. 5-127608 is to dynamically control the brightness of the light source depending on an input scene, as described above. However, an object is to make the display brightness constant. Therefore, floating blackness is not mitigated for a dark scene as in movie software. The brightness of the light source is controlled on the basis of the average value between the maximum value and the minimum value of the input video signal. Therefore, in an input scene where the maximum value is locally high or the minimum value is locally low, the features of the input scene cannot be entirely grasped, leading to an unsuitable control of the brightness of the light source. Further, the brightness of the light source is discontinuously controlled. Accordingly, the brightness of a screen is greatly changed at the time point where the brightness of the light source is changed, thereby giving an uncomfortable feeling to a viewer.
On the other hand, a technique proposed in Japanese Patent Laid-Open Publication No. 6-160811 is also to dynamically control the brightness of the light source depending on the input video signal, as described above. However, the brightness of the light source is controlled depending on the maximum value of the input video signal. In the case of such an input scene where the maximum value is locally high despite the fact that an average brightness (picture) level (hereinafter referred to as APL) is low, the brightness of the light source is high, causing floating blackness in a dark portion of video. Further, a discharging light source, such as a xenon lamp or a high-pressure mercury lamp, employed for the projection type display apparatus repeats a rapid change in driving conditions to cause instability of lighting starting characteristics, and degradation in stable lighting characteristics and degradation in life characteristics, such as flicker at the time of stationary lighting, resulting in the degraded reliability of the lamp.
Therefore, an object of the present invention is to mitigate the problems of the quality of display video (lack of a contrast feeling, floating blackness) in a video display apparatus including a transmission type or reflection type display component having a light modulating function and a light source for irradiating light onto the display component. Another object of the present invention is to mitigate the degradation in the reliability of a light source, an optical aperture, or a light control element for controlling the amount of light irradiated onto the display component in a case where the amount of light is dynamically controlled.
Still another object of the present invention is to provide a video display apparatus and a video display method, in which the brightness of a light source is changed depending on an input video signal, to realize the display of video having a contrast feeling, while capable of preventing an uncomfortable feeling during display caused by the rapid change in the brightness of the light source and improving the life characteristics of the light source.